This invention relates to a method for displaying multitone images derived from grayscale images having a number of shades of gray. Multitone images are images printed with a plurality of inks, preferably two, three or four inks. An image printed with two inks is called a "duotone"; three inks, a "tritone"; and four inks is called a "quadtone". Unlike four-color separations which are used to reproduce color images, multitones are used to reproduce grayscale images with multiple printing inks.
For simplicity, the multitone printing process of the invention will be described with respect to a duotone. The advantage of using a duotone printing process instead of printing with only a single black ink is that single ink processes provide limited grayscale dynamic range. Grayscale dynamic range is the total number of gradations in the printing color which can be printed.
The use of multiple inks increases the number of distinct grayscale gradations possible, thereby increasing the tonal range of the resulting printed image. Duotones were originally created with black and gray inks. The black ink was used to capture the detail in the shadows; the gray ink was used to represent the midtones and highlights. In modern printing processes, however, duotones often use a black ink and a colored ink such as a Pantone. This mixture provides the duotone with a slight tint in addition to the increase in tonal range provided by the duotone process.
Traditionally, duotones prints are difficult to create, requiring either a great deal of experience, extensive trial-and-error or both. Duotones are made by scanning a black and white photo and filtering the scan through two different tonal curves. The tonal curve used for the black ink typically removes the details in the highlight areas. The curve used for the colored ink usually eliminates the details in the shadows. There is no easy way to determine the correct transfer curves for a duotone because different curves are required depending on the characteristics of the photo and the color of the printing inks being used. It is desireable to make this determination using some kind of previewing technique.
A pre-press operator has two ways available to her to preview the duotone before going to press. If the film has been created directly from the scan, she can make a proof of the film, for example using Matchprint or Chromalin techniques, to gain an appreciation of the way the two films will print together. This is often difficult because proofing inks are not usually available in a wide variety of spot colors. Thus she may end up running proofs with a black ink and another color ink which only approximates the true color of the second ink.
If an operator has scanned the image into an electronic file, he can bring the image into a color electronic pre-press system (CEPS) and view the image as he would a traditionally scanned and separated CMYK (cyan, magenta, yellow and black) color image. With such a CEPS system, image data is maintained as four-channel information where the cyan, magenta, yellow and black channels are essentially four independent images which are electronically combined to display a composite color image on a computer monitor. If the operator is trying to simulate a duotone print created, for example, with black and red inks, he might erase the yellow and cyan channels, leaving only the black and magenta channels intact. This effectively simulates a duotone created with black and magenta inks from a separated CMYK image. Again, however, if he were printing with red (instead of magenta) ink, he would see only a fairly crude approximation of what the real duotone would look like.
Furthermore, the data for each printing color must be maintained independently. Once the composite image has been displayed, the relationships between the individual printing plates of each color and the original grayscale image from which they had been created is lost. For example, one could modify the black printing plate, such as by airbrushing, without making any modification to the colored plate. Once that was done, it is no longer possible to recreate the original image together with the relationships between the individual plates and the original grayscale image.
Accordingly, prior art techniques work by essentially using the original grayscale photo to create two new images, one black and one colored. These images are separate and distinct from the original grayscale image (whether they be in film or in electronic form), and lose their ability to be changed with reference to the original multitone image unless that original image is rescanned.
It has been an objective in duotone printing to have a technique for displaying multitone images derived from a grayscale image before printing, where the operator has the ability to make changes in the multitone layer individually while maintaining the original relationships between the layers.